Process for preparing formic acid by reaction of carbon dioxide with hydrogen

ABSTRACT

A process for preparing formic acid by reaction of carbon dioxide (1) with hydrogen (2) in a hydrogenation reactor (I) in the presence of
         a catalyst comprising an element of group 8, 9 or 10 of the Periodic Table,   a tertiary amine comprising at least 12 carbon atoms per molecule and   a polar solvent comprising one or more monoalcohols selected from among methanol, ethanol, propanols and butanols and also water,
 
to form formic acid/amine adducts as intermediates which are subsequently thermally dissociated, with work-up of the output (3) from the hydrogenation reactor (I) in a plurality of process steps, where
 
a tertiary amine-comprising stream (13) from the work-up is used as selective solvent for the catalyst, is proposed.

RELATED APPLICATIONS

This patent application claims the benefit of pending U.S. provisionalpatent application Ser. No. 61/359,405 filed Jun. 29, 2010 incorporatedin its entirety herein by reference.

BACKGROUND OF THE INVENTION

The invention relates to a process for preparing formic acid by reactionof carbon dioxide with hydrogen in a hydrogenation reactor in thepresence of a catalyst comprising an element of group 8, 9 or 10 of thePeriodic Table, a tertiary amine and a polar solvent to form formicacid/amine adducts as intermediates which are subsequently thermallydissociated.

Adducts of formic acid and tertiary amines can be thermally dissociatedinto free formic acid and tertiary amine and therefore serve asintermediates in the preparation of formic acid.

Formic acid is an important and versatile product. It is used, forexample, for acidification in the production of animal feeds, aspreservative, as disinfectant, as auxiliary in the textile and leatherindustry, as a mixture with its salts for deicing aircraft and runwaysand also as synthetic building block in the chemical industry.

The abovementioned adducts of formic acid and tertiary amines can beprepared in various ways, for example (i) by direct reaction of tertiaryamine with formic acid, (ii) by hydrolysis of methyl formate to formformic acid in the presence of the tertiary amine or (iii) by catalytichydration of carbon monoxide or hydrogenation of carbon dioxide to formformic acid in the presence of the tertiary amine. The latter process ofcatalytic hydrogenation of carbon dioxide has the particular advantagethat carbon dioxide is available in large quantities and is flexible interms of source.

An industrially promising process appears to be, in particular, thecatalytic hydrogenation of carbon dioxide in the presence of amines (W.Leitner, Angewandte Chemie 1995, 107, pages 2391 to 2405; P. G. Jessop,T. Ikariya, R. Noyori, Chemical Reviews 1995, 95, pages 259 to 272). Theadducts of formic acid and amines formed here can be thermallydissociated into formic acid and the amine used, which can berecirculated to the hydrogenation.

The catalyst necessary for the reaction comprises one or more elementsfrom group 8, 9 or 10 of the Periodic Table, i.e. Fe, Co, Ni, Ru, Rh,Pd, Os, Ir and/or Pt. The catalyst preferably comprises Ru, Rh, Pd, Os,Ir and/or Pt, particularly preferably Ru, Rh and/or Pd and veryparticularly preferably Ru.

To make an economical process possible, the catalyst used has to beseparated off ideally completely from the product stream andrecirculated to the reaction, for two reasons:

-   (1) large losses of the expensive catalyst would incur considerable    additional costs and would be prohibitive for economical operation    of the process.-   (2) in the thermal dissociation of the formic acid/amine adducts,    very little catalyst should be present since in the absence of a CO₂    and/or H₂ pressure this also catalyzes the reverse reaction and thus    leads to losses of the formic acid formed.

Decomposition of formic acid/amine adducts in the presence of a catalyst(x=0.4−3)

The transition metal-catalyzed decomposition of formic acid has beendescribed comprehensively, especially recently: C. Fellay, N. Yan, P. J.Dyson, G. Laurenczy Chem. Eur. J. 2009, 15, 3752-3760; C. Fellay, P. J.Dyson, G. Laurenczy Angew. Chem. 2008, 120, 4030-4032; B. Loges, A.Boddien, H. Junge, M. Beller Angew. Chem. 2008, 120, 4026-4029; F. JoóChemSusChem 2008, 1, 805-808; S. Enthaler ChemSusChem 2008, 1, 801-804;S. Fukuzumi, T. Kobayashi, T. Suenobu ChemSusChem 2008, 1, 827-834; A.Boddien, B. Loges, H. Junge, M. Beller ChemSusChem 2008, 1, 751-758.

The catalysts used here are in principle also suitable in principle forthe hydrogenation of CO₂ to formic acid (P. G. Jessop, T. Ikariya, R.Noyori Chem. Rev. 1995, 95, 259-272; P. G. Jessop, F. Joó, C. C. TaiCoord. Chem. Rev. 2004, 248, 2425-2442; P. G. Jessop, HomogeneousHydrogenation of Carbon Dioxide, in: The Handbook of HomogeneousHydrogenation, Hrsg.: J. G. de Vries, C. J. Elsevier, Volume 1, 2007,Wiley-VCH, pp. 489-511). Thus, the hydrogenation catalysts have to beseparated off before the thermal dissociation in order to prevent theundesirable decomposition of formic acid.

WO 2008/116,799 discloses a process for the hydrogenation of carbondioxide in the presence of a catalyst which comprises a transition metalof transition group VIII (groups 8, 9, 10) and is suspended orhomogeneously dissolved in a solution, a tertiary amine having at leastone hydroxyl group and a polar solvent to form an adduct of formic acidand the tertiary amine. The hydroxyl group(s) in the tertiary amineenable an increased carbon dioxide solubility compared to thetriethylamine which is usually used to be achieved. As preferredhomogeneous catalysts, mention may be made of RuH₂L₄ having monodentatephosphorus-based ligands L and RuH₂(LL)₂ having bidentatephosphorus-based ligands LL and particularly preferably RuH₂[P(C₆H₅)₃]₄.As polar solvents, mention may be made of alcohols, ethers, sulfolanes,dimethyl sulfoxide and amides whose boiling point at atmosphericpressure is at least 5° C. above that of formic acid. The tertiaryamines which are preferably to be used also have a boiling point abovethat of formic acid. Since no phase separation takes place, the work-upof the entire reaction product mixture is carried out by distillation,optionally after prior removal of the catalyst, in which the adduct offormic acid and the tertiary amine which is formed is thermallydissociated and the formic acid liberated is isolated as overheadproduct. The bottom product comprising tertiary amine, polar solvent andoptionally catalyst is recirculated to the hydrogenation stage.

A disadvantage of this process is the introduction of the entire liquidreaction product mixture into the apparatus for thermal dissociation anddistillation, optionally after prior specific removal of the homogeneouscatalyst by means of a separate process step, for example an extraction,adsorption or ultrafiltration step. The apparatus for the thermaldissociation and distillation consequently has to be made larger andmore complex both in terms of the higher liquid throughput and the morespecific separation properties, which is reflected, inter alia, in thecapital costs (for example via engineering input, material, spacerequirement). In addition, the higher liquid throughput also results ina higher energy usage.

However, the fundamental work on the catalytic hydrogenation of carbondioxide to form formic acid was carried out as early as the 1970s and1980s. The processes of BP Chemicals Ltd filed as the patents EP 0 095321 A, EP 0 151 510 A and EP 0 181 078 A may be considered to resulttherefrom. All three documents describe the hydrogenation of carbondioxide in the presence of a homogeneous catalyst comprising atransition metal of transition group VIII (groups 8, 9, 10), a tertiaryamine and a polar solvent to form an adduct of formic acid and thetertiary amine. As preferred homogeneous catalysts, EP 0 095 321 A andEP 0 181 078 A in each case make mention of ruthenium-based carbonyl-,halide- and/or triphenylphosphine-comprising complex catalysts and EP 0151 510 A mentions rhodium-phosphine complexes. Preferred tertiaryamines are C₁-C₁₀-trialkylamines, in particular the short-chainC₁-C₄-trialkylamines, and also cyclic and/or bridged amines such as1,8-diazabicyclo[5.4.0]undec-7-ene, 1,4-diazabicyclo[2.2.2]octane,pyridine or picolines. The hydrogenation is carried out at a carbondioxide partial pressure of up to 6 MPa (60 bar), a hydrogen partialpressure of up to 25 MPa (250 bar) and a temperature from about roomtemperature to 200° C.

EP 0 095 321 A and EP 0 151 510 A teach the use of an alcohol as polarsolvent. However, since primary alcohols tend to form formic esters(organic formates), secondary alcohols, in particular isopropanol, arepreferred. In addition, the presence of water is described asadvantageous. According to the examples in EP 0 095 321 A, the reactionproduct mixture is worked up by directly subsequent two-stagedistillation in which the low boilers alcohol, water, tertiary amine areseparated off in the first stage and the adduct of formic acid and thetertiary amine is separated off at the top under vacuum conditions inthe second stage. EP 0 151 510 A likewise teaches a work-up bydistillation, but with reference to EP 0 126 524 A with subsequentreplacement of the tertiary amine in the adduct which has been separatedoff by distillation by a weaker, less volatile nitrogen base beforethermal cleavage of the adduct in order to aid or make possible thesubsequent thermal dissociation to produce free formic acid.

EP 0 181 078 A teaches the targeted choice of the polar solvent on thebasis of three essential criteria which have to be fulfilled at the sametime:

-   (i) the homogeneous catalyst has to be soluble in the polar solvent;-   (ii) the polar solvent must not have an adverse effect on the    hydrogenation; and-   (iii) the adduct of formic acid and the tertiary amine which is    formed should be able to be readily separated off from the polar    solvent.

As particularly suitable polar solvents, mention is made of variousglycols and phenyipropanols.

According to the teaching of EP 0 181 078 A, the work-up of the reactionproduct mixture is carried out by firstly separating off the gaseouscomponents (in particular unreacted starting materials hydrogen andcarbon dioxide) at the top of an evaporator and separating off thehomogeneous catalyst dissolved in the polar solvent at the bottom andrecirculating them to the hydrogenation stage. The adduct of formic acidand the tertiary amine is subsequently separated off from the remainingliquid phase comprising the adduct of formic acid and the tertiaryamine, free tertiary amine and possibly water and the remaining part ofthe liquid phase comprising the free tertiary amine and possibly wateris recirculated to the hydrogenation stage. The separation can beeffected by distillation or phase separation of the two-phase system(decantation).

A further significant teaching of EP 0 181 078 A is the subsequent,absolutely necessary replacement of the tertiary amine in the adductwhich has been separated off by a weaker, less volatile nitrogen basebefore the adduct is thermally dissociated in order to aid or makepossible the subsequent thermal dissociation to produce free formicacid. As particularly suitable weaker nitrogen bases, mention is made ofimidazole derivatives such as 1-n-butylimidazole.

A disadvantage of the process of EP 0 181 078 A is the very complicated,four-stage work-up of the reaction product mixture by

-   (i) separating off the gaseous components and also the homogeneous    catalyst and the polar solvent in an evaporator and recirculating    them to the hydrogenation stage;-   (ii) separating off the adduct of formic acid and the tertiary amine    in a distillation column or a phase separator and recirculating the    remaining liquid stream to the hydrogenation stage;-   (iii) replacing the tertiary amine in the adduct of formic acid and    the tertiary amine by a weaker, less volatile nitrogen base in a    reaction vessel having a superposed distillation column and    recirculating the tertiary amine liberated to the hydrogenation    stage; and-   (iv) thermally dissociating the adduct of formic acid and the weaker    nitrogen base and recirculating the weaker nitrogen base liberated    to the base replacement stage.

A further, important disadvantage of the process of EP 0 181 078 A andalso of the processes of EP 0 095 321 A and EP 0 151 510 A is the factthat the adduct of formic acid and the tertiary amine partlyredissociates into carbon dioxide and hydrogen in the presence of thehomogeneous catalyst during the work-up in the evaporator. As a solutionto this problem, EP 0 329 337 A proposes the addition of a decompositioninhibitor which reversibly inhibits the homogeneous catalyst. Aspreferred decomposition inhibitors, mention is made of carbon monoxideand oxidants. However, disadvantages of this are the introduction offurther substances into the overall process and the necessity ofreactivating the inhibited homogeneous catalyst before it is usedfurther.

EP 0 357 243 A, too, addresses the disadvantage of the partialredissociation of the adduct of formic acid and the tertiary amine inthe process of EP 0 181 078 A by joint work-up of the reaction productmixture in the evaporator. The process proposed in EP 0 357 243 Ateaches the use of a homogeneous catalyst comprising a transition metalof transition group VIII (groups 8, 9, 10), a tertiary amine and twodifferent solvents, namely a nonpolar, inert solvent and a polar, inertsolvent, which form two immiscible liquid phases in the catalytichydrogenation of carbon dioxide to form an adduct of formic acid andtertiary amine. As nonpolar solvents, mention is made of aliphatic andaromatic hydrocarbons but also of phosphines having aliphatic and/oraromatic hydrocarbon radicals. Polar solvents mentioned are water,glycerol, alcohols, polyols, sulfolanes and mixtures thereof, with waterbeing preferred. The homogeneous catalyst dissolves in the nonpolarsolvent and the adduct of formic acid and tertiary amine dissolves inthe polar solvent. After the reaction is complete, the two liquid phasesare separated, for example by decantation, and the nonpolar phasecomprising the homogeneous catalyst and the nonpolar solvent isrecirculated to the hydrogenation stage. The polar phase comprising theadduct of formic acid and tertiary amine and the polar solvent is thensubjected to an absolutely necessary replacement of the tertiary aminein the adduct by a weaker, less volatile nitrogen base before thermaldissociation of the adduct in order to aid or make possible thesubsequent thermal dissociation to produce free formic acid. In a manneranalogous to EP 0 181 078 A, imidazole derivatives such as1-n-butylimidazole are also mentioned here as particularly suitableweaker nitrogen bases.

A disadvantage of the process of EP 0 357 243 A is the very complicated,three-stage work-up of the reaction product mixture by

-   (i) separating the two liquid phases and recirculating the phase    comprising the homogeneous catalyst and the nonpolar solvent to the    hydrogenation stage;-   (ii) replacing the tertiary amine in the adduct of formic acid and    the tertiary amine of the other phase by a weaker, less volatile    nitrogen base in a reaction vessel with superposed distillation    column and recirculating the tertiary amine liberated to the    hydrogenation stage; and-   (iii) thermally dissociating the adduct of formic acid and the    weaker nitrogen base and recirculating the weaker nitrogen base    liberated to the base replacement stage.

A further disadvantage of the process of EP 0 357 243 A is the use oftwo solvents and thus introduction of a further substance into theoverall process.

As an alternative, EP 0 357 243 A also discloses the possibility ofusing only one solvent. In this case, the addition of the polar solventin which the adduct of formic acid and the tertiary amine wouldotherwise dissolve is omitted. The sole solvent used here is thenonpolar solvent which dissolves the homogeneous catalyst. However, thisalternative also has the disadvantage of the very complicated,three-stage work-up as described above.

DE 44 31 233 A likewise describes the hydrogenation of carbon dioxide inthe presence of a catalyst comprising a transition metal of transitiongroup VIII (groups 8, 9, 10), a tertiary amine and a polar solvent andwater to form an adduct of formic acid and the tertiary amine, in which,however, the catalyst is present in heterogeneous form and the activecomponent is applied to a inert support. Preferred tertiary amines areC₁-C₈-trialkylamines, polyamines having from 2 to 5 amino groups,aromatic nitrogen heterocycles such as pyridine or N-methylimidazole andalso cyclic and/or bridged amines such as N-methylpiperidine,1,8-diazabicyclo[5.4.0]undec-7-ene or 1,4-diazabicyclo[2.2.2]octane. Assuitable polar solvents, mention is made of the low-boilingC₁-C₄-monoalcohols, and, in a manner analogous to EP 0 095 321 A,secondary alcohols are preferred. The hydrogenation is carried out at atotal pressure of from 4 to 20 MPa (from 40 to 200 bar) and atemperature of from 50 to 200° C. For the work-up of the adduct offormic acid and tertiary amine which is formed, DE 44 31 233 A teachesthe use of known methods with explicit reference to the work-up withreplacement of the tertiary amine in the adduct of formic acid and thetertiary amine by a weaker, less volatile nitrogen base as disclosed inEP 0 357 243 A. In a manner analogous to the process of EP 0 357 243 A,the process of DE 44 31 233 A also has the disadvantage of the verycomplicated, three-stage work-up of the reaction product mixture.

BRIEF SUMMARY OF THE INVENTION

It was an object of the present invention to provide a process forpreparing formic acid by hydrogenation of carbon dioxide, which does nothave the abovementioned disadvantages of the prior art or suffers fromthem only to a significantly reduced extent and which leads toconcentrated formic acid in a high yield and high purity. Furthermore,the process should be able to be carried out in a simple manner or atleast a simpler manner than is described in the prior art, in particularby means of a simpler process concept for working up the output from thehydrogenation reactor, simpler process stages, a reduced number ofprocess stages or simpler apparatuses. In addition, the process shouldalso be able to be carried out with the lowest possible consumption ofenergy.

The work-up of the output from the hydrogenation reactor should, inparticular, be carried out using exclusively the materials present inthe process, without additional auxiliaries, and these should be able tobe completely or largely completely recycled in the process.

The object is achieved by a process for preparing formic acid byreaction of carbon dioxide with hydrogen in a hydrogenation reactor inthe presence of

-   -   a catalyst comprising an element of group 8, 9 or 10 of the        Periodic Table,    -   a tertiary amine comprising at least 12 carbon atoms per        molecule and    -   a polar solvent comprising one or more monoalcohols selected        from among methanol, ethanol, propanols and butanols and also        water,        to form formic acid/amine adducts as intermediates which are        subsequently thermally dissociated,        where a tertiary amine which has a boiling point at least 5° C.        higher than that of formic acid and        two liquid phases are formed in the hydrogenation, namely a        lower phase which comprises predominantly the polar solvent and        in which the formic acid/amine adducts are present in enriched        form and an upper phase which comprises predominantly the        tertiary amine and in which the catalyst is present in enriched        form, wherein the work-up of the output from the hydrogenation        reactor is carried out by the following process steps:    -   1) separation of the two liquid phases from the hydrogenation        reactor in a phase separation vessel, recirculation of the upper        phase from the phase separation vessel to the hydrogenation        reactor and passing-on of the lower phase from the phase        separation vessel to an extraction apparatus in which    -   2) residues of catalyst are extracted with the same tertiary        amine which was used in the hydrogenation and the catalyst-laden        tertiary amine is recycled to the hydrogenation reactor and the        catalyst-free stream of polar solvent loaded with the formic        acid/amine adducts is passed on to a distillation unit in which    -   3) the polar solvent is separated off as overhead stream and        recycled to the hydrogenation reactor to give a stream which    -   4) is separated in a phase separation vessel into an upper phase        comprising predominantly the tertiary amine and a lower phase        comprising predominantly the formic acid/amine adducts, where    -   5) the lower phase from the phase separation vessel is fed to a        thermal dissociation unit and dissociated therein into a stream        which comprises the tertiary amine and is recirculated to the        phase separation vessel and pure formic acid and

-   a stream comprising the tertiary amine is conveyed from the phase    separation vessel into the extraction apparatus as selective solvent    for the catalyst.

A BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a block diagram of a preferred embodiment of theprocess of the invention.

FIG. 2 illustrates a block diagram of a further preferred embodiment ofthe process of the invention.

FIG. 3 illustrates a block diagram of an additional preferred embodimentof the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The catalyst to be used in the hydrogenation of carbon dioxide in theprocess of the invention is preferably a homogeneous catalyst. Itcomprises an element of group 8, 9 or 10 of the Periodic Table, i.e. Fe,Co, Ni, Ru, Rh, Pd, Os, Ir and/or Pt. The catalyst preferably comprisesRu, Rh, Pd, Os, Ir and/or Pt, particularly preferably Ru, Rh and/or Pd,and very particularly preferably Ru.

The abovementioned elements are homogeneously dissolved in the form ofcomplex-like compounds in the reaction mixture. The homogeneous catalystshould be selected so that it is present in enriched form together withthe tertiary amine in the liquid phase (B). For the present purposes,“in enriched form” means a partition coefficient of the homogeneouscatalystP=[concentration of homogeneous catalyst in liquid phase(B)]/[concentration of homogeneous catalyst in liquid phase (A)]of >1. The choice of the homogeneous catalyst is generally made by meansof a simple test in which the partition coefficient of the desiredhomogeneous catalyst under the planned process conditions is determinedexperimentally.

Liquid phase (A) is here the lower phase from the phase separationvessel in process step 1).

Owing to their good solubility in tertiary amines, preference is givento using a metal-organic complex comprising an element of group 8, 9 or10 of the Periodic Table and at least one phosphine group having atleast one unbranched or branched, acyclic or cyclic, aliphatic radicalhaving from 1 to 12 carbon atoms, where individual carbon atoms can alsobe substituted by >P—, as homogeneous catalysts in the process of theinvention. Branched cyclic aliphatic radicals thus also include radicalssuch as —CH₂—C₆H₁₁. Suitable radicals are, for example, methyl, ethyl,1-propyl, 2-propyl, 1-butyl, 2-butyl, 1-(2-methyl)propyl,2-(2-methyl)propyl, 1-pentyl, 1-hexyl, 1-heptyl, 1-octyl, 1-nonyl,1-decyl, 1-undecyl, 1-dodecyl, cyclopentyl, cyclohexyl, cycloheptyl andcyclooctyl, methylcyclopentyl, methylcyclohexyl, 1-(2-methyl)pentyl1-(2-ethyl)hexyl, 1-(2-propyl)heptyl and norbonyl. The unbranched orbranched, acyclic or cyclic, aliphatic radical preferably comprises atleast 1 and preferably not more than 10 carbon atoms. In the case of anexclusively cyclic radical in the abovementioned sense, the number ofcarbon atoms is from 3 to 12 and preferably at least 4 and alsopreferably not more than 8. Preferred radicals are ethyl, 1-butyl,sec-butyl, 1-octyl and cyclohexyl.

The phosphine group can comprise one, two or three of the abovementionedunbranched or branched, acyclic or cyclic, aliphatic radicals. These canbe identical or different. The phosphine group preferably comprisesthree of the abovementioned unbranched or branched, acyclic or cyclic,aliphatic radicals and particular preference is given to all threeradicals being identical. Preferred phosphines are P(n-C_(n)H_(2n+1))₃where n is from 1 to 10, particularly preferably tri-n-butylphosphine,tri-n-octylphosphine and 1,2-bis(dicyclohexylphosphino)ethane.

As mentioned above, individual carbon atoms can also be substitutedby >P— in the abovementioned unbranched or branched, acyclic or cyclic,aliphatic radicals. Polydentate, for example bidentate or tridentate,phosphine ligands are thus also comprised. These preferably comprise thegroup or >P—CH₂CH₂—P< or

If the phosphine group comprises radicals other than the abovementionedunbranched or branched, acyclic or cyclic, aliphatic radicals, thesegenerally correspond to those which are otherwise customarily used inphosphine ligands for metal-organic complex catalysts. Examples whichmay be mentioned are phenyl, tolyl and xylyl.

The metal-organic complex can comprise one or more, for example two,three or four, of the abovementioned phosphine groups having at leastone unbranched or branched, acyclic or cyclic, aliphatic radical. Theremaining ligands of the metal-organic complex can have various natures.Examples which may be mentioned are hydride, fluoride, chloride,bromide, iodide, formate, acetate, propionate, carboxylate,acetylacetonate, carbonyl, DMSO, hydroxide, trialkylamine, alkoxide.

The homogeneous catalysts can either be used directly in their activeform or be generated only under reaction conditions from customarystandard complexes such as [M(p-cymene)Cl₂]₂, [M(benzene)Cl₂]_(n),[M(COD)(allyl)], [MCI₃×H₂O], [M(acetylacetonate)₃], [M(DMSO)₄Cl₂] whereM is an element of group 8, 9 or 10 of the Periodic Table by addition ofthe corresponding phosphine ligand or ligands.

Homogeneous catalysts which are preferred in the process of theinvention are [Ru(P^(n)Bu₃)₄(H)₂], [Ru(P^(n)octyl₃)₄(H)₂],[Ru(P^(n)Bu₃)₂(1,2-bis(dicyclohexylphosphino)-ethane)(H)₂],[Ru(P^(n)octyl₃)₂(1,2-bis(dicyclohexylphosphino)ethane)(H)₂],[Ru(PEt₃)₄(H)₂. By means of these, TOF (turnover frequency) values ofgreater than 1000 h⁻¹ can be achieved in the hydrogenation of carbondioxide.

When homogeneous catalysts are used, the amount of the specified metalcomponent in the metal-organic complex which is used is generally from0.1 to 5000 ppm by weight, preferably from 1 to 800 ppm by weight andparticularly preferably from 5 to 800 ppm by weight, in each case basedon the total liquid reaction mixture in the hydrogenation reactor.

The partition coefficient of the homogeneous catalyst based on theamount of ruthenium in the amine phase and the product phase comprisingthe formic acid/amine adduct is in the range of P greater than 0.5,preferably greater than 1.0 and particularly preferably greater than 4,after the hydrogenation.

The tertiary amine to be used in the hydrogenation of carbon dioxide inthe process of the invention has a boiling point which is at least 5° C.higher than that of formic acid. Here, as is customary, the boilingpoint of compounds have to be based on the same pressure in each casewhen the relative position of boiling points is indicated. The tertiaryamine is selected or matched to the polar solvent so that the tertiaryamine is present in enriched form in the upper phase in thehydrogenation reactor. For the present purposes, “in enriched form”means a proportion by weight of >50% of the free, i.e. not bound in theform of the formic acid/amine adduct, tertiary amine in the upper phase,based on the total amount of free, tertiary amine in the two liquidphases. The proportion by weight is preferably >90%. The tertiary amineis generally selected by means of a simple test in which the solubilityof the desired tertiary amine in the two liquid phases under the plannedprocess conditions is determined experimentally. The upper phase canadditionally comprise amounts of the polar solvent and of a nonpolarinert solvent.

The tertiary amine to be used preferably has a boiling point which is atleast 10° C. higher, particularly preferably at least 50° C. higher andvery particularly preferably at least 100° C. higher, than that offormic acid. A restriction in terms of an upper limit to the boilingpoint is not necessary since a very low vapor pressure of the tertiaryamine is in principle an advantage for the process of the invention. Ingeneral, the boiling point of the tertiary amine at a pressure of 1013hPa abs, if necessary at a pressure extrapolated by known methods fromvacuum to 1013 hPa abs, is below 500° C.

The tertiary amine which is preferably to be used in the process of theinvention is an amine, comprising at least 12 carbon atoms per molecule,of the general formula (Ia)NR¹R²R³  (Ia),where the radicals R¹ to R³ are identical or different and are each,independently of one another, an unbranched or branched, acyclic orcyclic, aliphatic, araliphatic or aromatic radical having in each casefrom 1 to 16 carbon atoms, preferably from 1 to 12 carbon atoms, whereindividual carbon atoms can also be substituted, independently of oneanother, by a hetero group selected from the group consisting of —O—and >N— or two or all three radicals can also be joined to one anotherto form a chain comprising at least four atoms in each case.

Examples of suitable amines are:

-   Tri-n-butylamine, tri-n-pentylamine, tri-n-hexylamine,    tri-n-heptylamine, tri-n-octylamine, tri-n-nonylamine,    tri-n-decylamine, tri-n-undecylamine, tri-n-dodecylamine,    tri-n-tridecylamine, tri-n-tetradecylamine, tri-n-pentadecylamine,    tri-n-hexadecylamine, tri(2-ethylhexyl)amine.-   Dimethyldecylamine, dimethyldodecylamine, dimethyltetradecylamine,    ethyldi(2-propyl)amine (bp_(1013 hPa)=127° C.), dioctylmethylamine,    dihexylmethylamine.-   Tricyclopentylamine, tricyclohexylamine, tricycloheptylamine,    tricyclooctylamine and derivatives thereof which are substituted by    one or more methyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 2-butyl or    2-methyl-2-propyl groups.-   Dimethylcyclohexylamine, methyldicyclohexylamine,    diethylcyclohexylamine, ethyldicyclohexylamine,    dimethylcyclopentylamine, methyldicyclopentylamine.-   Triphenylamine, methyldiphenylamine, ethyldiphenylamine,    propyldiphenylamine, butyldiphenylamine, 2-ethylhexyldiphenylamine,    dimethylphenylamine, diethylphenylamine, dipropylphenylamine,    dibutylphenylamine, bis(2-ethylhexyl)phenylamine, tribenzylamine,    methyldibenzylamine, ethyldibenzylamine and derivatives thereof    which are substituted by one or more methyl, ethyl, 1-propyl,    2-propyl, 1-butyl, 2-butyl or 2-methyl-2-propyl groups.-   N—C₁-C₁₂-alkylpiperidines, N,N-di-C₁-C₁₂-alkylpiperazines,    N—C₁-C₁₂-alkylpyrrolidines, N—C₁-C₁₂-alkylimidazoles and derivatives    thereof which are substituted by one or more methyl, ethyl,    1-propyl, 2-propyl, 1-butyl, 2-butyl or 2-methyl-2-propyl groups.-   1,8-Diazabicyclo[5.4.0]undec-7-ene (“DBU”),    1,4-diazabicyclo[2.2.2]octane (“DABCO”),    N-methyl-8-azabicyclo[3.2.1]octane (“tropane”),    N-methyl-9-azabicyclo[3.3.1]nonane (“granatane”),    1-azabicyclo[2.2.2]octane (“quinuclidine”).

It is naturally also possible to use mixtures of any amount of varioustertiary amines in the process of the invention.

In the process of the invention, particular preference is given to usingan amine of the general formula (Ia) in which the radicals R¹ to R³ areselected independently from the group consisting of C₁-C₁₂-alkyl,C₅-C₈-cycloalkyl, benzyl and phenyl as tertiary amine.

Particular preference is given to using a saturated amine, i.e. onlycomprising single bonds, of the general formula (Ia) as tertiary aminein the process of the invention.

Very particular preference is given to using an amine of the generalformula (Ia) in which the radicals R¹ to R³ are selected independentlyfrom the group consisting of C₅-C₈-alkyl, in particulartri-n-pentylamine, tri-n-hexylamine, tri-n-heptylamine,tri-n-octylamine, dimethylcyclohexylamine, methyldicyclohexylamine,dioctylmethylamine and dimethyldecylamine, as tertiary amine in theprocess of the invention.

In particular, an amine of the general formula (Ia) in which theradicals R¹ to R³ are selected independently from among C₅- and C₆-alkylis used as tertiary amine.

The tertiary amine is preferably present in liquid form in all processstages of the process of the invention. However, this is not absolutelynecessary. It would also be sufficient for the tertiary amine to be atleast dissolved in suitable solvents. Suitable solvents are in principlethose which are chemically inert in respect of the hydrogenation ofcarbon dioxide and the thermal dissociation of the adduct and in whichthe tertiary amine and, if a homogeneous catalyst is used, also thelatter readily dissolve but do not readily dissolve the polar solventand the formic acid/amine adduct. Possibilities are therefore inprinciple chemically inert, nonpolar solvents such as aliphatic,aromatic or araliphatic hydrocarbons, for example octane and higheralkanes, toluene, xylenes.

The polar solvent to be used in the hydrogenation of carbon dioxide inthe process of the invention has a boiling point which is at least 5° C.lower than the temperature required for the dissociation of the formicacid/amine adducts at the same pressure. The polar solvent is to beselected or matched to the tertiary amine so that the polar solvent ispresent in enriched form in the lower phase. For the present purposes,“in enriched form” means a proportion by weight of >50% of the polarsolvent in the lower phase based on the total amount of polar solvent inthe two liquid phases. The proportion by weight is preferably >70%. Thepolar solvent is generally selected by means of a simple test in whichthe solubility of the desired polar solvent in the two liquid phasesunder the planned process conditions is determined experimentally.

The polar solvent can be a pure polar solvent or a mixture of variouspolar solvents, as long as the abovementioned conditions in respect ofboiling point and phase behavior which the solvent has to meet aresatisfied.

The polar solvent to be used preferably has a boiling point which is atleast 10° C. lower, particularly preferably at least 50° C. lower, thanthe temperature required for dissociation of the formic acid/amineadducts at the same pressure. In the case of solvent mixtures, theboiling points of the solvent mixture used or an azeotrope orheteroazeotrope are at least 10° C. lower, particularly preferably atleast 50° C. lower, than the temperature required for dissociation ofthe formic acid/amine adducts at the same pressure.

Classes of substances which are suitable as polar solvents arepreferably alcohols and the formic esters thereof and water. Thealcohols have a boiling point which is at least 10° C. lower,particularly preferably at least 50° C. lower, than the temperaturerequired for dissociation of the formic acid/amine adducts at the samepressure in order to keep the esterification of the alcohols by formicacid very low.

As suitable alcohols, mention may be made of alcohols in the case ofwhich the trialkylammonium formates preferentially dissolve in a mixtureof the alcohol with water and this product phase has a miscibility gapwith the free trialkylamine. Suitable alcohols are, for example,methanol, ethanol, 2-methoxyethanol, 1-propanol, 2-propanol, 1-butanol,2-butanol, 2-methyl-1-propanol. The ratio of alcohol to water has to beselected so that together with the trialkylammonium formate and thetrialkylamine a two-phase mixture in which the major part of thetrialkylammonium formate, the water and the polar solvent are present inthe lower phase is formed, which is generally determined by means of asimple test in which the solubility of the desired polar solvent mixturein the two liquid phases under the planned process conditions isdetermined experimentally.

The molar ratio of the polar solvent or solvent mixture to be used inthe process of the invention to the tertiary amine used is generallyfrom 0.5 to 30 and preferably from 1 to 20.

The carbon dioxide to be used in the hydrogenation of carbon dioxide canbe used in solid, liquid or gaseous form. It is also possible to useindustrially available gas mixtures comprising carbon dioxide as long asthese are largely free of carbon monoxide (proportion by volume of <1%of CO). The hydrogen to be used in the hydrogenation of carbon dioxideis generally gaseous. Carbon dioxide and hydrogen can also compriseinert gases such as nitrogen or noble gases. However, the content ofthese is advantageously below 10 mol % based on the total amount ofcarbon dioxide and hydrogen in the hydrogenation reactor. Althoughlarger amounts may likewise be tolerable, they generally require the useof a higher pressure in the reactor which in turn makes furthercompression energy necessary.

The hydrogenation of carbon dioxide is preferably carried out in theliquid phase at a temperature of from 20 to 200° C. and a total pressureof from 0.2 to 30 MPa abs. The temperature is preferably at least 30° C.and particularly preferably at least 40° C. and also preferably not morethan 150° C., particularly preferably not more than 120° C. and veryparticularly preferably not more than 100° C. The total pressure ispreferably at least 1 MPa abs and particularly preferably at least 5 MPaabs and also preferably not more than 30 MPa abs.

The partial pressure of carbon dioxide is generally at least 0.5 MPa andpreferably at least 2 MPa and also generally not more than 8 MPa. Thepartial pressure of hydrogen is generally at least 0.5 MPa andpreferably at least 1 MPa and also generally not more than 25 MPa andpreferably not more than 10 MPa.

The molar ratio of hydrogen to carbon dioxide in the feed to thehydrogenation reactor is preferably from 0.1 to 10 and particularlypreferably from 1 to 3.

The molar ratio of carbon dioxide to tertiary amine in the feed to thehydrogenation reactor is generally from 0.1 to 10 and preferably from0.5 to 3.

As hydrogenation reactors, it is in principle possible to use allreactors which are suitable in principle for gas/liquid reactions at thegiven temperature and the given pressure. Suitable standard reactors forliquid-liquid reaction systems are indicated, for example, in K. D.Henkel, “Reactor Types and Their Industrial Applications”, in Ullmann'sEncyclopedia of Industrial Chemistry, 2005, Wiley-VCH Verlag GmbH & Co.KGaA, DOI: 10.1002/14356007.b04_(—)087, chapter 3.3 “Reactors forgas-liquid reactions”. Examples which may be mentioned are stirred tankreactors, tube reactors or bubble column reactors.

The hydrogenation of carbon dioxide in the process of the invention canbe carried out batchwise or continuously. In the case of batchoperation, the reactor is charged with the desired liquid and optionallysolid starting materials and auxiliaries and carbon dioxide and hydrogenare subsequently introduced to the desired pressure at the desiredtemperature. After the reaction is complete, the reactor is generallydepressurized and the two liquid phases which are formed are separatedfrom one another. In the continuous mode of operation, the startingmaterials and auxiliaries including the carbon dioxide and hydrogen areintroduced continuously. Accordingly, the liquid phase is continuouslydischarged from the reactor so that the average liquid level in thereactor remains constant. Preference is given to the continuoushydrogenation of carbon dioxide.

The average residence time in the hydrogenation reactor is generallyfrom 10 minutes to 5 hours.

The formic acid/amine adducts formed in the hydrogenation of carbondioxide in the presence of the catalyst to be used, the polar solventand the tertiary amine generally have the general formula (IIa)NR¹R²R³·x_(i)HCOOH  (IIa),where the radicals R¹ to R³ correspond to the radicals described for thetertiary amine (Ia) and x_(i) is from 0.4 to 5, preferably from 0.7 to1.6. The respective average values of the amine-formic acid ratios inthe product phases in the respective process steps, i.e. the factorx_(i), can be determined, for example, by determining the formic acidcontent by titration with an alcoholic KOH solution againstphenolphthalein and the amine content can be determined by gaschromatography. The composition of the formic acid/amine adducts, i.e.the factor x_(i), can change during the various process steps. Thus, forexample, adducts having a relatively high formic acid content with x₂>x₁and x₂=1 to 4 are generally formed after removal of the polar solvent,with the excess, free amine being able to form a second phase.

Two liquid phases are formed in the hydrogenation of carbon dioxide bythe process of the invention. The lower phase is enriched with theformic acid/amine adducts and the polar solvent. With regard to theformic acid/amine adducts, “enriched” means a partition coefficient ofthe formic acid/amine adductsP=[concentration of formic acid/amine adduct (II) in liquid phase(A)]/[concentration of formic acid/amine adduct (II) in liquid phase(B)]of >1. The partition coefficient is preferably 2 and particularlypreferably 5. The upper phase is enriched with the tertiary amine. If ahomogeneous catalyst is used, this is likewise present in enriched formin the upper phase.

The liquid phases (A) and (B) have the meaning defined above.

The two liquid phases formed are, in the process of the invention,separated from one another and the upper phase is recirculated to thehydrogenation reactor. Recirculation of a further liquid phasecomprising unreacted carbon dioxide present in addition to the twoabovementioned liquid phases and also of a gas phase comprisingunreacted carbon dioxide and/or unreacted hydrogen to the hydrogenationreactor may also be advantageous. It may also be desirable, for exampleto discharge undesirable by-products or impurities, to discharge part ofthe upper phase and/or part of the carbon dioxide or liquid or gaseousphases comprising carbon dioxide and hydrogen from the process.

The two liquid phases are generally separated by gravimetric phaseseparation. As phase separation vessels, it is possible to use, forexample, standard apparatuses and standard methods which are described,for example, in E. Müller et al., “Liquid-Liquid Extraction”, inUllmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH VerlagGmbH & Co. KGaA, DOI:10.1002/14356007.b03_(—)06, chapter 3 “Apparatus”.In general, the liquid phase enriched with the formic acid/amine adductsand the polar solvent is heavier and forms the lower phase.

The phase separation can be effected, for example, by depressurization,preferably to about or close to atmospheric pressure, and cooling of theliquid reaction mixture, for example to about or close to ambienttemperature. However, there is a risk that at least part of the gasdissolved in the liquid phases at the higher reaction pressure, inparticular carbon dioxide, will degas during the depressurization andhave to be compressed separately as a gas stream and recirculated to thehydrogenation reactor. Likewise, the lower phase has to be broughtseparately to the reaction pressure before recirculation to thehydrogenation reactor. Here, a suitable compressor designed according tothe pressure difference to be overcome or a pump, which also consumesadditional energy in operation, has to be provided for each of the gasand liquid phases to be recirculated.

In the context of the present invention, it has been found that in thecase of the present system, i.e. a lower phase enriched with the formicacid/amine adducts and the polar solvent and an upper phase enrichedwith the tertiary amine and in the case of the use of a homogeneouscatalyst also with this, the two liquid phases can separate very wellfrom one another even at a significantly elevated pressure. For thisreason, the solvent in the process of the invention is preferablyselected so that the separation of the lower phase enriched in theformic acid/amine adducts and the polar solvent from the upper phase inwhich with the tertiary amine and also the recirculation of the upperphase to the hydrogenation reactor can be carried out at a pressure offrom 1 to 30 MPa abs. Depending on the total pressure in thehydrogenation reactor, the pressure is preferably not more than 30 MPaabs. It can even be possible to separate the two liquid phases from oneanother without prior depressurization and recirculate the upper phaseto the hydrogenation reactor without an appreciable pressure increase.In this case, and also in the case of an only slight depressurization,it is possible to entirely dispense with recirculation of any gas phase.Whether this omission is possible for the respective specific systemshould be determined beforehand in the case of doubt by simpleexperimental examples.

The process of the invention can therefore preferably be carried out insuch a way that the pressure and the temperature in the hydrogenationreactor and in the phase separation vessel are the same or approximatelythe same; for the present purposes, the same means a pressure differenceof up to +/−5 bar or a temperature difference of up to +/−5° C.

In one embodiment the phase separation is carried out at a pressure ofat least 50%, very particularly preferably at least 90% and inparticular at least 95%, of the reaction pressure. The pressure in thephase separation is particularly preferably not more than 105% and veryparticularly preferably not more than 100% of the reaction pressure.

It has surprisingly also been found that in the case of the presentsystem the two liquid phases separate very readily from one another evenat an elevated temperature corresponding to the reaction temperature. Inthis case, no cooling is necessary for the phase separation and nosubsequent heating of the upper phase to be recirculated is required,which likewise saves energy.

The major part of the polar solvent of the lower phase which has beenseparated off is separated off thermally from the formic acid/amineadducts in a distillation unit, with the polar solvent removed bydistillation being recirculated to the hydrogenation reactor. The pureformic acid/amine adducts and free amine are obtained at the bottom ofthe distillation unit, since when the polar solvent is removed formicacid/amine adducts having a relatively low amine content are formed, asa result of which a two-phase bottoms mixture comprising an amine phaseand a formic acid/amine adduct phase can be formed.

The thermal removal of the polar solvent or mixture, see above, ispreferably carried out at a temperature at the bottom at which, at thegiven pressure, no free formic acid is formed from the formic acid/amineadduct having the higher (x1) or lower (x2) amine content. In general,the temperature at the bottom of the thermal separation unit is at least20° C., preferably at least 50° C. and particularly preferably at least70° C., and generally not more than 210° C., preferably not more than190° C. The pressure is generally at least 1 hPa abs, preferably atleast 50 hPa abs and particularly preferably at least 100 hPa abs, andgenerally not more than 1 MPa abs and preferably 0.1 MPa abs.

The thermal removal of the polar solvent or mixture is carried outeither in an evaporator or in a distillation unit comprising vaporizerand column filled with ordered packing, random packing elements and/ortrays. The solvent can be condensed after the thermal separation, withthe enthalpy of condensation liberated once again being able to beutilized for, for example, preheating the solvent coming withamine/formic acid adduct mixture coming from the extraction toevaporation temperature.

As an alternative, only parts of the solvent mixture can be separatedoff. This applies in particular in the case of solvent components whichcan be separated off via a side stream in the later formic aciddistillation. (Keyword: aqueous formic acid).

The formic acid/amine adducts which are obtained after the thermalremoval of the polar solvent or mixture or parts of the solvent are thendissociated thermally into free formic acid and free tertiary amine in adistillation unit, with the free formic acid formed being removed bydistillation and the free tertiary amine comprised in the bottoms fromthe distillation unit being recirculated to the hydrogenation reactor.Here, the free amine obtained as second phase in the thermal removal ofthe polar solvent can be separated off beforehand in a phase separationvessel, in a joint phase separation vessel be fed together with thebottom product from the thermal dissociation unit to the formic acidremoval or as two-phase mixture directly to the dissociation unit (seegeneral embodiments). The formic acid liberated can be taken off, forexample, (i) at the top, (ii) at the top and as side offtake stream or(iii) only as side offtake stream. If formic acid is taken off at thetop, a formic acid purity of up to 99.99% by weight is possible. Whenformic acid is taken off as side offtake stream, aqueous formic acid isobtained, with a mixture comprising about 85% by weight of formic acidbeing of particular importance here in industrial practice. Depending onthe water content of the feed to the distillation unit, the majority ofthe formic acid is taken off as overhead product or as side product. Ifnecessary, it is even possible to take off formic acid only as sideproduct, in which case the required amount of water may be deliberatelyadded. The thermal dissociation of the formic acid/amine adduct isgenerally carried out under the process parameters known from the priorart in respect of pressure, temperature and configuration of theapparatus. Thus, for example, reference may be made to the descriptionsin EP 0 181 078 A or WO 2006/021,411. The distillation unit to be usedgenerally comprises a distillation column which generally comprisesrandom packing elements, ordered packings and/or trays.

In general, the temperature at the bottom of the distillation column isat least 130° C., preferably at least 140° C. and particularlypreferably at least 150° C., and generally not more than 210° C.,preferably not more than 190° C. and particularly preferably not morethan 185° C. The pressure is generally at least 1 hPa abs, preferably atleast 50 hPa abs and particularly preferably at least 100 hPa abs, andgenerally not more than 500 hPa abs, preferably not more than 300 hPaabs and particularly preferably not more than 250 hPa abs.

A water-comprising stream of formic acid is optionally taken off as sideproduct. In the case of addition of water, for example to promote thehydrogenation, this is even particularly advantageous.

The solution of the adduct of tertiary amine and formic acid isextracted with streams of free tertiary amine coming from theappropriate phase separation vessels and recirculated to thehydrogenation reactor. This occurs in order to separate residual amountsof hydrogenation catalyst from the product stream. Without thisextraction, hydrogenation catalyst could get into the apparatus for thethermal dissociation of the adduct of tertiary amine and formic acid,catalyze the decomposition of formic acid and thus reduce the yield offormic acid. Residual amounts of hydrogen and carbon dioxide aredisposed of as offgas.

The extraction is carried out at temperatures of from 30 to 100° C. andpressures of from 1 to 80 bar. The extraction can also be carried outunder hydrogen pressure.

The extraction of the hydrogenation catalyst can be carried out in anysuitable apparatus known to those skilled in the art, preferably incountercurrent extraction columns, mixer-settler cascades orcombinations of mixer-settlers with columns.

The product stream from the extraction apparatus, which comprises asolution of the formic acid/amine adduct in the respective solvent orsolvent mixture, is fed into the thermal dissociation unit in order toseparate the polar solvent or solvent mixture from the formic acid/amineadduct. The phase comprising tertiary amine and hydrogenation catalystfrom the extraction apparatus is recirculated to the hydrogenationstage.

Apart from the catalyst, amounts of individual components of the polarsolvent from the liquid phase to be extracted are sometimes dissolved inthe extractant, viz. the amine stream. This is not a disadvantage forthe process since the amount of solvent already extracted does not haveto be fed to the solvent removal and may thus save vaporization energy.

It can be advantageous to integrate an apparatus for the adsorption oftraces of hydrogenation catalyst between the extraction apparatus andthe thermal separation apparatus. Numerous adsorbents are suitable forthe adsorption. Examples are polyacrylic acid and salts thereof,sulfonated polystyrenes and salts thereof, activated carbons,montmorillonites, bentonites, silica gels and also zeolites.

If the amount of hydrogenation catalyst in the product stream from thephase separation vessel B is less than 1 ppm, in particular less than0.1 ppm, the adsorption apparatus is sufficient for separating off andrecovering the hydrogenation catalyst. The extraction stage can then beomitted and the tertiary amine can be recirculated together with theorganic solvent to the hydrogenation stage.

Compared to the integrated processes which have previously beendescribed in EP 181 078 B1 and EP 357 243 B1, the process of theinvention has a series of advantages: the same tertiary amine is usedfor binding of the formic acid in the hydrogenation and thermaldissociation of the formic acid/amine adducts. This amine, which isobtained in free form in the thermal dissociation, is then used forextraction of catalyst residues from the product phase in order torecirculate traces of the catalyst together with the amine to thereaction vessel. It has a higher stability than the previously describedN-alkylimidazoles. There are virtually no losses of noble metals. Thecatalyst is prevented from getting into the thermal dissociation unitand catalyzing the decomposition of formic acid therein. It is a greatadvantage that the catalyst is separated off in its active form and canbe recirculated. High formic acid yields and a high product purity areachieved. Extractions and phase separations replace two distillations.This enables energy and capital costs to be reduced. In addition, thefirst phase separation can be carried out under superatmosphericpressure, which results in smaller offgas streams.

The invention will be illustrated below with the aid of examples and adrawing.

EXAMPLES Examples A-1 to A-17 (Hydrogenation and Phase Separation, i.e.Process Step 1 of the Work-Up of the Output From the HydrogenationReactor (I))

A 250 ml autoclave made of Hastelloy C and provided with a magneticstirrer bar was charged under inert conditions with tertiary amine,polar solvent and homogeneous catalyst. The autoclave was subsequentlyclosed and CO₂ was injected at room temperature. H₂ was then injectedand the reactor was heated while stirring (700 rpm). After the desiredreaction time, the autoclave was cooled and the reaction mixture wasdepressurized. Unless indicated otherwise, a two-phase product mixturewas obtained, with the upper phase being enriched with the still freetertiary amine and the homogeneous catalyst and the lower phase beingenriched with the polar solvent and the formic acid/amine adduct formed.The total content of formic acid in the formic acid/amine adduct wasdetermined by potentiometric titration with 0.1 N KOH in MeOH using a“Mettler Toledo DL50” titrator. The turnover frequency (=TOF; for thedefinition of the TOF see: J. F. Hartwig, Organotransition MetalChemistry, 1st edition, 2010, University Science Books,Sausalito/California p. 545) and the reaction rate was calculated. Thecomposition of the two phases was determined by gas chromatography. Theruthenium content was determined by atomic absorption spectroscopy(=AAS). The parameters and results of the individual experiments areshown in Tables 1.1 to 1.5.

Examples A-1 to A-17 show that, in the process of the invention, high tovery high reaction rates of up to 0.98 mol kg⁻¹ h⁻¹ are achieved just byvarying the tertiary amine, the polar solvent, the catalyst in respectof the ligands and the metal component, the amount of catalyst and theamount of water added. All systems examined formed two phases, with theupper phase in each case being enriched in the still free tertiary amineand the homogeneous catalyst and the lower phase in each case beingenriched in the polar solvent and the formic acid/amine adduct formed.

TABLE 1.1 Example A-1 Example A-2 Example A-3 Example A-4 Tertiary amine75 g of trihexylamine 75 g of trihexylamine 75 g of tripentylamine 75 gof tripentylamine Polar solvent (used) 17.8 g of 1-propanol 21.7 g of2-propanol 17.8 g of 1-propanol 17.8 g of 1-propanol 7.3 g of water 3.3g of water 7.3 g of water 7.3 g of water Catalyst 0.2 g of[Ru(P^(n)Bu₃)₄(H)₂] 0.2 g of [Ru(P^(n)Bu₃)₄(H)₂] 0.3 g of[Ru(P^(n)Oct₃)₄(H)₂] 0.2 g of [Ru(P^(n)Bu₃)₄(H)₂] Injection of CO₂ 19.6g to 2.4 MPa abs 20.0 g to 2.3 MPa abs 20.0 g to 2.5 MPa abs 20.3 g to2.5 MPa abs Injection of H₂ To 10.4 MPa abs To 10.3 MPa abs To 10.6 MPaabs To 10.5 MPa abs Heating To 50° C. To 50° C. To 50° C. To 50° C.Pressure change To 10.0 MPa abs To 10.5 MPa abs To 11.4 MPa abs To 11.5MPa abs Reaction time 1 hour 1 hour 1 hour 1 hour Peculiarity — — — —Upper phase 57.5 g 62.3 g 75.6 g 63.8 g 8.0% of 1-propanol 15.4% of2-propanol 10.9% of 1-propanol 5.7% of 1-propanol 0.9% of water 1.7% ofwater 0.9% of water 0.5% of water 91.1% of trihexylamine 82.9% oftrihexylamine 88.2% of tripentylamine 93.8% of tripentylamine Lowerphase 43.6 g 37.3 g 22.9 g 38.4 g 5.9% of formic acid 4.7% of formicacid 3.4% of formic acid 6.8% of formic acid 30.3% of 1-propanol 32.4%of 2-propanol 41.7% of 1-propanol 36.7% of 1-propanol 15.6% of water5.9% of water 28.9% of water 18.2% of water 48.3% of trihexylamine 56.8%of trihexylamine 26% of tripentylamine 38.3% of tripentylamine k_(Ru)(c_(Ru) in upper phase/ 1.60 1.02 85 2.7 c_(Ru) in lower phase) TOF 252h⁻¹ 175 h⁻¹ 81 h⁻¹ 250 h⁻¹ Reaction rate 0.54 mol kg⁻¹ h⁻¹ 0.38 mol kg⁻¹h⁻¹ 0.17 mol kg⁻¹ h⁻¹ 0.56 mol kg⁻¹ h⁻¹

TABLE 1.2 Example A-5 Example A-6 Example A-7 Example A-8 Tertiary amine75 g of tripentylamine 75 g of tripentylamine 75 g of tripentylamine 75g tripentylamine Polar solvent (used) 21.8 g of 2-propanol 17.8 g of1-propanol 17.8 g of 1-propanol 18.8 g of methanol 3.3 g of water 7.3 gof water 7.3 g of water 6.3 g of water Catalyst 0.2 g of 0.2 g of 0.2 gof 0.2 g of [Ru(P^(n)Bu₃)₄(H)₂] [Ru(P^(n)Bu₃)₄(H)₂], 0.2 g[Ru(P^(n)Bu₃)₄(H)₂], 0.2 g [Ru(P^(n)Bu₃)₄(H)₂] of 1,2-bis-dicyclohexyl-of 1,2-bis-dicyclohexyl- phosphinoethane phosphinoethane Injection ofCO₂ 20.1 g to 2.4 MPa abs 20.2 g to 2.8 MPa abs 20.0 g to 2.5 MPa abs20.0 g to 2.3 MPa abs Injection of H₂ To 10.4 MPa abs To 8.1 MPa abs To10.5 MPa abs To 10.3 MPa Heating To 50° C. To 50° C. To 50° C. To 50° C.Pressure change To 11.0 MPa abs To 8.5 MPa abs To 10.9 MPa abs. To 10.5MPa Reaction time 1 hour 1 hour 1 hour 1 hour Peculiarity — — — — Upperphase 65.7 g 60.6 g 51.4 g 60.5 g 11.5% of 2-propanol 5.1% of 1-propanol3.9% of 1-propanol 3.1% of methanol 1.0% of water 94.9% oftripentylamine 96.1% of tripentylamine 96.9% of tripentylamine 87.5 oftripentylamine Lower phase 35.0 g 40.8 g 50.1 g 40.8 g 5.6% of formicacid 7.0% of formic acid 8.4% of formic acid 7.3% of formic acid 40.6%of 2-propanol 36.0% of 1-propanol 31.5% of 1-propanol 41.5% of methanol7.5% of water 17.9% of water 14.6% of water 15.4% of water 46.3% oftripentylamine 39.1% of tripentylamine 45.5% of tripentylamine 35.8% oftripentylamine k_(Ru) (c_(Ru) in upper phase/ 1.4 2.2 2.5 4.8 c_(Ru) inlower phase) TOF 195 h⁻¹ 282 h⁻¹ 413 h⁻¹ 290 h⁻¹ Reaction rate 0.43 molkg⁻¹ h⁻¹ 0.61 mol kg⁻¹ h⁻¹ 0.90 mol kg⁻¹ h⁻¹ 0.64 mol kg⁻¹ h⁻¹

TABLE 1.3 Example A-9 Example A-10 Example A-11 Example A-12 Tertiaryamine 70 g of trihexylamine 75 g of tripentylamine 75 g oftripentylamine 75 g of trihexylamine Polar solvent (used) 15.0 g ofethanol 21.5 g of methanol 24.0 g of methanol 22.0 g of methanol 5.0 gof water 3.6 g of water 1.0 g of water 3.0 g of water Catalyst 0.2 g of[Ru(P^(n)Bu₃)₄(H)₂] 0.2 g of [Ru(P^(n)Bu₃)₄(H)₂] 0.16 g of[Ru(P^(n)Oct₃)₄(H)₂] 0.16 g of [Ru(P^(n)Oct₃)₄(H)₂], 0.08 g of 1,2-bis(dicyclohexylphosphino)- ethane Injection of CO₂ 20.2 g to 2.5 MPaabs 20.0 g to 2.2 MPa abs 19.9 g to 2.3 MPa abs 20.0 g to 2.5 MPa absInjection of H₂ To 10.9 MPa abs To 10.5 MPa abs To 10.3 MPa abs To 10.5MPa abs Heating To 50° C. To 50° C. To 50° C. To 50° C. Pressure changeTo 11.8 MPa abs To 10.3 MPa abs To 10.2 MPa abs To 10.6 MPa abs Reactiontime 1 hour 1 hour 1 hour 1 hour Peculiarity — — — — Upper phase 52.0 g49.5 g 63.3 g 46.7 g 4.6% of ethanol 2.5% of methanol 8.4% of methanol4.1% of methanol 95.4% of trihexylamine 77.5 g of tripentylamine 91.6%of tripentylamine 95.9% of trihexylamine Lower phase 38.4 g 52.8 g 35.9g 54.2 g 7.3% of formic acid 8.7% of formic acid 4.5% of formic acid7.2% of formic acid 32.8% of ethanol 38.5% of methanol 52.1% of methanol37.1% of methanol 13.0% of water 6.8% of water 2.8% of water 5.5% ofwater 46.9% of tripentylamine 46.0% of tripentylamine 40.7% oftripentylamine 50.2% of trihexylamine k_(Ru) (c_(Ru) in upper phase/ 1.92.5 14.0 1.7 c_(Ru) in lower phase) TOF 271 h⁻¹ 446 h⁻¹ 343 h⁻¹ 806 h⁻¹Reaction rate 0.67 mol kg⁻¹ h⁻¹ 0.98 mol kg⁻¹ h⁻¹ 0.35 mol kg⁻¹ h⁻¹ 0.84mol kg⁻¹ h⁻¹

TABLE 1.4 Example A-13 Example A-14 Example A-15 Example A-16 Tertiaryamine 75 g of trihexylamine 75 g of trihexylamine 75 g of trihexylamine75 g of trihexylamine Polar solvent (used) 24.0 g of methanol 25.0 g ofethanol 25.0 g of 1-propanol 25.0 g of ethanol 6.7 g of water 6.0 g ofwater 6.0 g of water 8.0 g of water Catalyst 0.18 g of 0.16 g of 0.16 gof 0.16 g of [Ru(PnBu3)4(H)2] [Ru(PnOct3)4(H)2], [Ru(PnOct3)4(H)2],[Ru(PnOct3)4(H)2], 0.08 g of 1,2- 0.08 g of 1,2- 0.08 g of 1,2-bis(dicyclohexyl- bis(dicyclohexyl- bis(dicyclohexyl- phosphino)ethanephosphino)ethane phosphino)ethane Injection of CO₂ 20.2 g to 3.5 MPa abs19.9 g to 2.5 MPa abs 19.9 g to 2.5 MPa abs 19.5 g to 2.6 MPa absInjection of H₂ To 11.5 MPa abs To 11.5 MPa abs To 10.5 MPa abs To 10.7MPa abs Heating To 50° C. To 50° C. To 50° C. To 50° C. Pressure changeTo 11.0 MPa abs To 10.5 MPa abs To 11.3 MPa abs To 11.6 MPa abs Reactiontime 1 hour 1 hour 1 hour 2 hours Peculiarity — — Upper phase 44.1 g60.2 g 46.4 g 48.9 g 2.7% of methanol 0.7% of water 2.0% of water 0.6%of water 97.3% of trihexylamine 6.6% of ethanol 6.2% of 1-propanol 4.5%of ethanol 92.7% of trihexylamine 91.8% of trihexylamine 94.9% oftrihexylamine Lower phase 65.9 g 51.3 g 60.6 g 61.9 g 7.5% of formicacid 5.3% of formic acid 5.6% of formic acid 6.0% of formic acid 10.2%of water 9.3% of water 8.4% of water 12.4% of water 34.6% of methanol41.0% of ethanol 36.5% of 1-propanol 36.8% of ethanol 47.7% oftrihexylamine 44.4% of trihexylamine 49.5% of trihexylamine 44.8% oftrihexylamine k_(Ru) (c_(Ru) in upper phase/ 1.9 1.6 1.5 3.2 c_(Ru) inlower phase) TOF 551 h⁻¹ 569 h⁻¹ 726 h⁻¹ 351 h⁻¹ Reaction rate 0.98 molkg⁻¹ h⁻¹ 0.53 mol kg⁻¹ h⁻¹ 0.68 mol kg⁻¹ h⁻¹ 0.37 mol kg⁻¹ h⁻¹

TABLE 1.5 Example A-17 Tertiary amine 75 g of trihexylamine Polarsolvent (used) 25.0 g of 1-propanol 8.0 g of water Catalyst 0.16 g of[Ru(P^(n)Oct₃)₄(H)₂], 0.08 g of 1,2-bis(dicyclohexyl- phosphino)ethaneInjection of CO₂ 20.3 g to 2.5 MPa abs Injection of H₂ To 10.5 MPa absHeating To 50° C. Pressure change To 11.4 MPa abs Reaction time 2 hoursPeculiarity — Upper phase 37.1 g 2.6% of water 3.9% of 1-propanol 93.5%of trihexylamine Lower phase 74.5 g 6.2% of formic acid 9.4% of water31.5% of 1-propanol 52.9% of trihexylamine k_(Ru) (c_(Ru) in upperphase/ 3.9 c_(Ru) in lower phase) TOF 437 h⁻¹ Reaction rate 0.45 molkg⁻¹ h⁻¹

Examples B1-3 (Extraction of the Catalyst)

A 100 ml autoclave made of Hastelloy C and provided with a blade stirrerwas charged under inert conditions with the trialkylamine, polar solventand the catalyst. The autoclave was subsequently closed and CO₂ wasinjected at room temperature. H₂ was then injected and the reactor washeated while stirring (1000 rpm). After the reaction time, the autoclavewas cooled and the reaction mixture was depressurized. A two-phaseproduct mixture was obtained, with the upper phase being enriched in thestill free tertiary amine and the homogeneous catalyst and the lowerphase being enriched in the polar solvent and the formic acid/amineadduct formed. The lower phase was separated off and admixed three timesunder inert conditions with the same amount (mass of amine correspondsto the mass of the lower phase) of free trialkylamine (stirring for 10minutes at room temperature and subsequently separating the phases) forextraction of the catalyst. The total content of formic acid in theformic acid/amine adduct was determined by potentiometric titration with0.1 N KOH in MeOH using a “Mettler Toledo DL50” titrator. The rutheniumcontent was determined by AAS. The parameters and results of theindividual experiments are shown in Table 1.6.

Examples B-1 to B-3 show that, in the process of the invention, theamount of ruthenium catalyst in the product phase can be reduced byextraction with tertiary amine obtained in process step 5). This valuecould be reduced further by means of further extraction steps or acontinuous countercurrent extraction.

TABLE 1.6 Example B-1 Example B-2 Example B-3 Tertiary amine 37.5 g oftrihexylamine 37.5 g of tripentylamine 37.5 g of trihexylamine Polarsolvent (used) 12.0 g of methanol 10.0 g of methanol 10.0 g of methanol0.5 g of water 2.5 g of water 2.5 g of water Catalyst 0.16 g of[Ru(PnOctyl₃)₄(H)₂] 0.1 g of [Ru(PnButyl₃)₄(H)₂] 0.1 g of[Ru(PnButyl₃)₄(H)₂] Injection of CO₂ To 1.7 MPa abs To 2.3 MPa abs To2.5 MPa abs Injection of H₂ To 8.0 MPa abs To 8.0 MPa abs To 8.0 MPa absHeating 50° C. 50° C. 50° C. Reaction time 1.5 hours 1 hour 1 hour Upperphase 23.3 g 31.1 g 22.9 g Lower phase 26.2 g 17.4 g 27.5 g 6.1% offormic acid 5.6% of formic acid 7.2% of formic acid c_(Ru) in upperphase after 350 ppm 320 ppm 250 ppm reaction c_(Ru) in lower phase after33 ppm 80 ppm 170 ppm reaction c_(Ru) in lower phase after 21 ppm 50 ppm75 ppm extraction

Examples C1-C4 (Thermal Separation of the Polar Solvent From theTrialkylamine/Solvent/Formic Acid Mixtures Present as Product PhaseAfter the Extraction)

Alcohol and water are distilled off from the product phase (comprisesthe formic acid/amine adduct) under reduced pressure by means of arotary evaporator. A two-phase mixture (trialkylamine phase and formicacid/amine adduct phase) is formed as bottoms, the two phases areseparated and the formic acid content of the lower phase is determinedby potentiometric titration with 0.1 N KOH in MeOH using a “MettlerToledo DL50” titrator. The amine content and alcohol content aredetermined by gas chromatography. The parameters and results of theindividual experiments are shown in Table 1.7.

Examples C-1 to C-4 show that, in the process of the invention, variouspolar solvents can be separated off from the product phase under mildconditions to form a lower phase which is relatively rich in formic acidand an upper phase which comprises predominantly tertiary amine.

TABLE 1.7 Example C-1 Example C-2 Example C-3 Example C-4 Feed mixture(% by 18.7 g 19.3 g 81.8 g 88.6 g weight) 7.2% of formic acid 5.8% offormic acid 7.3% of formic acid 9.2% of formic acid 26.4% of 1-propanol22.8% of 2-propanol 41.3% of methanol 31.4% of ethanol 15.5% of water4.1% of water 15.4% of water 11.3% of water 48.3% of 67.2% of 35.9% of48.1% of trihexylamine trihexylamine tripentylamine tripentylamineFormic acid:amine feed 1:1.2 1:2.0 1:1 1:1.1 mixture Pressure 20 mbar 20mbar 200 mbar 200 mbar Temperature 50° C. 50° C. 100° C. 110° C. Formicacid content of 16.4% 18.0% 23.7% 22.7% lower phase after distillation(% by weight) Formic acid:amine in 1:0.76 1:0.78 1:0.6 1:0.56 lowerphase after distillation (molar ratio) Recovery of formic acid 95.3%93.7% 90.4% 95.2% after distillation

Examples D1 and D2 (Thermal Separation of the Polar Solvent From theTrialkylamine/Solvent/Formic Acid Mixtures and Dissociation of theFormic Acid/Amine Adduct)

Alcohol and water are distilled off from the product phase (comprisesthe formic acid/amine adduct) under reduced pressure by means of arotary evaporator. A two-phase mixture (trialkylamine phase and formicacid/amine adduct phase) is formed as bottoms and the two phases areseparated. The composition of the distillates (comprising the major partof the methanol and of the water), of the upper phase (comprising thefree trialkylamine) and of the lower phase (comprising the formicacid/amine adduct) was determined by gas chromatography and bypotentiometric titration of the formic acid against 0.1 N KOH in MeOHusing a “Mettler Toledo DL50” titrator. The formic acid is thenthermally dissociated from the trialkylamine in the lower phase from thefirst step in a vacuum distillation apparatus comprising a 10 cm Vigreuxcolumn. After the formic acid has been completely split off, asingle-phase bottom product comprising the pure trialkylamine isobtained and can be used for extraction of the catalyst andrecirculation to the hydrogenation. The formic acid and residual waterare present in the distillate. The composition of the bottoms and of thedistillate was determined by gas chromatography and by potentiometrictitration of the formic acid against 0.1 N KOH in MeOH using a “MettlerToledo DL50” titrator. The parameters and results of the individualexperiments are shown in Table 1.8.

Examples D-1 and D-2 show that, in the process of the invention, variouspolar solvents can be separated off from the product phase under mildconditions, with a lower phase which is relatively rich in formic acidand an upper phase comprising predominantly tertiary amine being formed.The formic acid can then be dissociated from the trialkylamine in thislower phase which is relatively rich in formic acid at elevatedtemperatures giving the free trialkylamine. The formic acid obtained inthis way still comprises a little water, but this can be separated offfrom the formic acid by means of a column having a relatively highseparation power. The trialkylamine obtained both in the removal of thesolvent and in the thermal dissociation can be used for removing thecatalyst from the product stream in process step 2).

TABLE 1.8 Example D-1a Example D-1b Example D-2b (removal of the polar(dissociation of the formic Example D-2a (dissociation of the formicsolvent) acid/amine adduct) (removal of the polar solvent) acid/amineadduct) Feed mixture 199.8 g Lower phase from D1-a 199.8 g Lower phasefrom D2-a (% by weight) 8.9% of formic acid 7.8% of formic acid 28.4% ofmethanol 33.0% of methanol 5.6% of water 15.1% of water 57.1% oftrihexylamine 44.0% of trihexylamine Formic acid:amine in 1:1.1 1:0.641:1 1:0.89 feed mixture Pressure 200 mbar 90 mbar 200 mbar 90 mbarTemperature 120° C. 153° C. 120° C. 153° C. Lower phase in the 79.8 g63.6 g 69.4 g 55.5 g bottoms after distillation 22.1% of formic acid100% of trihexylamine 14.9% of formic acid 99.7% of trihexylamine (% byweight) 1.5% of water 6.9% of water 0.3% of water 76.4% of trihexylamine78.2% of trihexylamine Upper phase in the 50.5 g Single-phase 32.7 gSingle-phase bottoms after distillation 100% of trihexylamine 99.7% oftrihexylamine 0.3% of water Distillate 66.6 g 14.9 g 93.1 g 12.9 g 0.3%of formic acid 92.1% of formic acid 70.1% of methanol 85.0% of formicacid 81.2% of methanol 7.9% of water 29.9% of water 15% of water 18.5%of water

The drawings specifically show:

FIG. 1 a block diagram of a preferred embodiment of the process of theinvention,

FIG. 2 a block diagram of a further preferred embodiment of the processof the invention and

FIG. 3 a block diagram of an additional preferred embodiment of theprocess of the invention.

In the embodiment as per FIG. 1, carbon dioxide, stream 1, and hydrogen,stream 2, are fed into the hydrogenation reactor I. In the reactor, thetwo streams are reacted in the presence of a catalyst comprising anelement of group 8, 9 or 10 of the Periodic Table, a tertiary amine anda polar solvent to give a formic acid/amine adduct. Two liquid phasesare formed here. The lower phase is enriched in the formic acid/amineadducts and the polar solvent, and the upper phase is enriched in thetertiary amine and, when a homogeneous catalyst is used, this too. Thetwo liquid phases are fed to a phase separator II (stream 3) andseparated from one another. The upper phase 4 is recirculated to thehydrogenation reactor I. The lower phase 5 is fed to an extractionapparatus III in which catalyst residues are extracted with the tertiaryamine from the phase separation vessel V. The tertiary amine togetherwith the catalyst residues (stream 6) from the extraction unit III isthen recirculated to the hydrogenation reactor I. The product phase 7from the extraction unit III is fed to the thermal separation unit IV inorder to separate off the polar solvent thermally from the formicacid/amine adducts. Stream 7 can be conveyed beforehand through anadsorbent bed in order to remove last traces of ruthenium from thisstream. The polar solvent which is separated off thermally in the unitIV is recirculated as stream 8 to the hydrogenation reactor I and thetwo-phase mixture from the bottom of the thermal separation unit IV,comprising the formic acid/amine adducts and the tertiary amine (stream9), is fed to the phase separation vessel V. The formic acid/amineadducts are separated off in the phase separation vessel V and fed asstream 10 to the distillation unit VI in which this stream 10 isthermally dissociated into free formic acid and tertiary amine. The freeformic acid is, for example, removed as overhead product, stream 12. Thetwo-phase bottoms from the distillation unit VI, comprising tertiaryamine and undissociated formic acid/amine adducts, stream 11, is fedback to the phase separation vessel V. The tertiary amine which isseparated off in the phase separation vessel V is fed to the extractionunit III in order to extract catalyst residues.

In the embodiment as per FIG. 2, carbon dioxide 1 and hydrogen 2 are fedinto the hydrogenation reactor I. In this reactor, they are reacted inthe presence of a catalyst comprising an element of group 8, 9 or 10 ofthe Periodic Table, a tertiary amine and a polar solvent to give formicacid/amine adducts. Two liquid phases are formed here. The lower phaseis enriched in the formic acid/amine adducts and the polar solvent, andthe upper phase is enriched with the tertiary amine and, when ahomogeneous catalyst is used, this too. The two liquid phases (stream 3)are fed to a phase separator II and separated from one another. Theupper phase is recirculated to the hydrogenation reactor I (stream 4).The lower phase is fed to an extraction apparatus III in which catalystresidues are extracted with the tertiary amine from the phase separationvessel V. The tertiary amine together with the catalyst residues fromthe extraction unit III is then recirculated to the hydrogenationreactor I (stream 6). The product phase 7 from the extraction unit IIIis fed to the thermal separation unit IV in order to separate off thepolar solvent thermally from the formic acid/amine adducts. Stream 7 canbe passed beforehand through an adsorbent bed in order to remove lasttraces of catalyst from this stream. The polar solvent which isthermally separated off in the unit IV is recirculated as stream 8 tothe hydrogenation reactor I and the two-phase mixture from the bottom ofthe thermal separation unit IV, comprising the formic acid/amine adductsand the tertiary amine, stream 9, is fed as stream 10 to thedistillation unit VI. In this, the formic acid/amine adducts arethermally dissociated into free formic acid and tertiary amine. The freeformic acid is, for example, removed as overhead product, stream 12. Thetwo-phase bottoms from the distillation unit VI, stream 11, comprisingtertiary amine and undissociated formic acid/amine adducts, are fed tothe phase separation vessel V. The tertiary amine which is separated offin the phase separation vessel V is fed as stream 13 to the extractionunit III in order to extract catalyst residues. Part of stream 13 canalso be recirculated directly to the hydrogenation reactor if not all ofthe amine is required for the extraction.

In the embodiment as per FIG. 3, carbon dioxide 1 and hydrogen 2 are fedto the hydrogenation reactor I. In this reactor, they are reacted in thepresence of a catalyst comprising an element of 8, 9 or 10 of thePeriodic Table, a tertiary amine and a polar solvent to give formicacid/amine adducts. Two liquid phases are formed here. The lower phaseis enriched in the formic acid/amine adducts and the polar solvent, andthe upper phase is enriched in the tertiary amine and, when ahomogeneous catalyst is used, this too. The two liquid phases are fed asstream 3 to a phase separator II and separated from one another. Theupper phase, stream 4, is recirculated to the hydrogenation reactor I.The lower phase, stream 5, is fed to an extraction apparatus III inwhich catalyst residues are extracted with the tertiary amine from thephase separation vessel IV and V. The tertiary amine together with thecatalyst residues from the extraction unit III is then recirculated tothe hydrogenation reactor I. The product phase 7 from the extractionunit III is fed to the thermal separation unit V in order to separateoff the polar solvent thermally from the formic acid/amine adducts.Stream 7 can be conveyed beforehand through an adsorbent bed in order toremove last traces of ruthenium from this stream. The polar solventwhich is separated off thermally in the unit IV is recirculated to thehydrogenation reactor I (stream 8) and the two-phase mixture from thebottom of the distillation unit IV, comprising the formic acid/amineadducts and the tertiary amine, stream 9, is fed to the phase separationvessel VII. The tertiary amine which is separated off in the phaseseparation vessel VII is fed to the extraction unit III. The formicacid/amine adducts from the phase separation vessel VII are fed to thedistillation unit VI. In this, the formic acid/amine adducts arethermally dissociated into free formic acid and tertiary amine. The freeformic acid is, for example, removed as overhead product, stream 12. Thetwo-phase bottoms from the distillation unit VI, comprising tertiaryamine and undissociated formic acid/amine adducts, stream 11, are fed tothe phase separation vessel V. The tertiary amine which is separated offin the phase separation vessel V is fed as stream 13 to the extractionunit III in order to extract catalyst residues.

The process of the invention makes it possible to obtain concentratedformic acid in high yield and high purity by hydrogenation of carbondioxide. In particular, it provides a particularly simple mode ofoperation which compared to the prior art has a simpler process concept,simpler process stages, a smaller number of process stages and simplerapparatuses. Thus, for example, if the tertiary amine and the polarsolvent are appropriately selected in the case of the use of ahomogeneous catalyst, the latter is separated off from formic acid/amineadducts by phase separation and recirculated without further work-upsteps to the hydrogenation reactor. The phase separation can also becarried out under superatmospheric pressure. The prompt separation ofthe catalyst from the formic acid/amine adducts formed suppresses abackreaction with decomposition into carbon dioxide and hydrogen. Inaddition, losses of catalyst and thus losses of noble metal areminimized by the retention or removal of the catalyst as a result of theformation of two liquid phases. In addition, catalyst remaining in theproduct stream can be virtually completely recirculated to thehydrogenation reactor as a result of the extraction with the free aminefrom the thermal dissociation units, which further minimizes the lossesof noble metal and is a very great advantage for an economical processand also largely suppresses the decomposition of the formic acid in thethermal dissociation units. Furthermore, no complicated separate basereplacement is required in the process of the invention, so that theformic acid/amine adducts formed in the hydrogenation reactor can beused directly for the thermal dissociation. The use of a low-boilingpolar solvent makes it possible for this to be separated off thermallyunder mild conditions in a stage preceding the thermal dissociation ofthe formic acid, as a result of which the esterification of alcoholsused and decomposition of the formic acid are minimized, a lower energyconsumption is necessary and a higher purity of the formic acid can beachieved. The simpler process concept makes it possible for theproduction plant required for carrying out the process of the inventionto be made more compact in the sense of a smaller space requirement andthe use of fewer apparatuses compared to the prior art. It has a lowercapital cost requirement and a lower energy consumption.

The invention claimed is:
 1. A process for preparing formic acid whichcomprises reacting carbon dioxide with hydrogen in a hydrogenationreactor in the presence of a catalyst comprising an element of group 8,9 or 10 of the Periodic Table, a tertiary amine comprising at least 12carbon atoms per molecule and a polar solvent comprising one or moremonoalcohols selected from among methanol, ethanol, propanols andbutanols and also water, to form formic acid/amine adducts asintermediates which are subsequently thermally dissociated, where atertiary amine which has a boiling point at least 5° C. higher than thatof formic acid and two liquid phases are formed in the reaction in thehydrogenation reactor, namely a lower phase which comprisespredominantly the polar solvent and in which the formic acid/amineadducts are present in enriched form and an upper phase which comprisespredominantly the tertiary amine and in which the catalyst is present inenriched form, wherein the work-up of the output from the hydrogenationreactor is carried out by the following process steps: 1) separating thetwo liquid phases from the hydrogenation reactor in a phase separationvessel, recirculation of the upper phase from the phase separationvessel to the hydrogenation reactor and passing-on of the lower phasefrom the phase separation vessel to an extraction apparatus in which 2)extracting residues of catalyst with the same tertiary amine which wasused in the hydrogenation and the catalyst-laden tertiary amine isrecycled to the hydrogenation reactor and the catalyst-free stream ofpolar solvent loaded with the formic acid/amine adducts is passed on toa distillation unit in which 3) separating off the polar solvent asoverhead stream and recycling to the hydrogenation reactor to give astream which 4) is separated in a phase separation vessel into an upperphase comprising predominantly the tertiary amine and a lower phasecomprising predominantly the formic acid/amine adducts, where 5) thelower phase from the phase separation vessel is fed to a thermaldissociation unit and dissociated therein into a stream which comprisesthe tertiary amine and is recirculated to the phase separation vesseland pure formic acid and a stream comprising the tertiary amine isconveyed from the phase separation vessel into the extraction apparatusas selective solvent for the catalyst.
 2. The process according to claim1, wherein an amine of the general formula (Ia)NR¹R²R³  (Ia) where the radicals R¹ to R³ are identical or different andare each, independently of one another, an unbranched or branched,acyclic or cyclic, aliphatic, araliphatic or aromatic radical having ineach case from 1 to 6 carbon atoms, where individual carbon atoms canalso be substituted, independently of one another, by a hetero groupselected from the group consisting of O and >N or two or all threeradicals can also be joined to one another to form a chain comprising atleast four atoms in each case, is used as tertiary amine, with theproviso that the tertiary amine comprises at least 12 carbon atoms permolecule.
 3. The process according to claim 2, wherein the radicals R¹to R³ are selected independently from the group consisting ofC1-C12-alkyl, C5-C8-cycloalkyl, benzyl and phenyl.
 4. The processaccording to claim 2, wherein a saturated amine of the general formula(Ia) is used as tertiary amine.
 5. The process according to claim 2,wherein the radicals R¹ to R³ are selected independently from C5 andC6-alkyl.
 6. The process according to claim 1, wherein methanol and/orethanol are used as polar solvent.
 7. The process according to claim 5,wherein methanol and/or ethanol are used as polar solvent.
 8. Theprocess according to claim 1, wherein the polar solvent comprises up to50% by weight of water.
 9. The process according to claim 1, wherein thepolar solvent comprises up to 50% by weight of water.
 10. The processaccording to claim 1, wherein the catalyst is a homogeneous catalyst.11. The process according to claim 10, wherein the homogeneous catalystis a metal-organic complex comprising an element of group 8, 9 or 10 ofthe Periodic Table and at least one phosphine group having at least oneunbranched or branched, acyclic or cyclic aliphatic radical having from1 to 12 carbon atoms, with individual carbon atoms also being able to besubstituted by >P—.
 12. The process according to claim 1, wherein thereaction in the hydrogenation reactor is carried out at a temperature inthe range from 20 to 200° C. and a pressure in the range from 0.2 to 30MPa abs.
 13. The process according to claim 12, wherein the pressure inthe hydrogenation reactor and in the phase separation vessel is the sameor approximately the same.
 14. The process according to claim 12,wherein the temperature in the hydrogenation reactor and in the phaseseparation vessel is the same.
 15. The process according to claim 13,wherein the temperature in the hydrogenation reactor and in the phaseseparation vessel is the same.